Carbon dioxide is a valuable industrial product having a variety of uses that require that carbon dioxide to be of high purity, namely, over 95 percent pure carbon dioxide by volume. In some instances, it is necessary to remove undesirable impurities from carbon dioxide. And in some instances, it is desirable not to lose valuable components in the product carbon dioxide. One important use for carbon dioxide is in the field of enhanced oil recovery in which the carbon dioxide is injected down hole in an oil field to drive oil to producing wells. Typically, in an enhanced oil recovery operation, a fresh stream of carbon dioxide is mixed with a recycle stream of carbon dioxide that is generated when oil is produced. This recycled carbon dioxide stream contains about 80 to 95 percent by volume carbon dioxide and the remainder, impurities that consist mainly of hydrocarbons ranging from C1 to C7 alkanes. In this regard, the methane content of the impurities is known to affect the performance of the enhanced oil recovery and hence, methane removal is beneficial for such operations.
As indicated above, carbon dioxide has many uses beyond enhanced oil recovery and can be introduced into a pipeline for use at locations that are situated a distance from the production site at which the carbon dioxide is produced. In such an application, moisture within the carbon dioxide stream can cause corrosion problems for the pipeline. Hence, moisture removal is important. Moreover, it is also beneficial to remove other impurities from a carbon dioxide stream to be injected into a pipeline because many uses of carbon dioxide require nearly pure carbon dioxide. Moreover, compressing a stream into a pipeline that contains impurities, that will invariably need to be removed, increases the electrical power costs associated with compressing the stream due to the added volume of the impurities.
Carbon dioxide can be produced by oxy-fuel combustion in, for example, coal-fired power plants. Typically, the flue gas produced by the combustion has a carbon dioxide purity that ranges from about 70 percent to about 90 percent depending upon the purity of the oxygen supplied and any air leakage into the boiler. Thus, the impurities within a carbon dioxide stream produced by such a process can include oxygen, nitrogen, argon, SOx and NOx. Sulfur oxides and nitrogen oxides are particularly undesirable impurities in any process stream. In enhanced oil recovery processes, the oxygen content must be less than 100 ppm and preferably less than 10 ppm and the desired purity of the carbon dioxide must be at minimum, about 95 percent pure. Hence, removal of the oxygen impurity is particularly important for enhanced oil recovery operations.
Carbon dioxide can also be produced from a hydrogen plant in which a hydrocarbon containing stream is subjected to steam methane reforming or alternatively to partial oxidation to produce a hydrogen, carbon monoxide, carbon dioxide and water containing stream known as synthesis gas. In any such plant, the synthesis gas can in turn be subjected to a water-gas shift reaction to react steam with the carbon monoxide and thereby increase the hydrogen and carbon dioxide contained in such a stream. In a plant designed to produce hydrogen, the hydrogen is typically separated from the synthesis gas by pressure swing adsorption. The waste stream produced by pressure swing adsorption contains carbon monoxide and methane impurities. These components are typically recovered and can be used to meet part of the fuel requirements of the plant. The carbon dioxide can be recovered from the synthesis gas either before or after the water-gas shift reactor.
In U.S. Pat. No. 6,301,927, an auto-refrigerated process for separating carbon dioxide is provided in which a carbon dioxide containing feed stream is compressed, cooled and expanded in a turboexpander so that it is partially liquefied and then introduced into a phase separator. The liquid component is then introduced into a stripping column to produce a liquid carbon dioxide product. The stripping column is reboiled by a compressed part of the feed stream that is at a much higher temperature than column operational temperatures. Such an operation results in thermal inefficiency that represents an irreversible loss that must be made up by increased refrigeration and ultimately, in increased compression and power requirements.
In U.S. Pat. No. 4,441,900, a process for removing carbon dioxide from natural gas is disclosed in which the feed stream is partially condensed by cooling and then is separated in a phase separator. The liquid stream resulting from the phase separation is subcooled and then fed to a stripping-distillation column to recover carbon dioxide-rich liquid and a first methane-rich vapor. The first methane-rich vapor is combined with the vapor from the phase separation and the combined vapor is further cooled and then rectified in a rectification column to produce a second carbon dioxide liquid and a second methane-rich vapor. Refrigeration is supplied by expanding the second methane-rich vapor and the carbon dioxide-rich liquid streams. All of the columns described in this patent are operated at the feed stream pressure and hence, reboiling within the columns is conducted at a relatively high temperature. As a result, the feed stream can supply only a limited amount of thermal energy to reboilers of the distillation columns and the carbon dioxide product streams will always contain significant amounts of methane.
U.S. Pat. No. 3,130,026 discloses another process for separating carbon dioxide from natural gas in which the feed is cooled and partially liquefied and then fed to a phase separator. The liquid stream from the phase separator is stripped in a stripping column. The carbon dioxide liquid stream from the stripping column is vaporized and then work expanded to recover power and to generate refrigeration which is used to partially liquefy the feed stream. Due to expansion of carbon dioxide in the separation process, carbon dioxide from the separation process is obtained at near atmospheric pressure. The expansion of carbon dioxide at subatmospheric temperature generates much lower power than that required for compressing carbon dioxide above atmospheric pressure. As a result, when compressed carbon dioxide is a desired product, this process will require significant energy for compression. Additionally, the use of multiple turboexpanders also increases the capital cost of the separation system.
U.S. Pat. No. 4,762,543 discloses a carbon dioxide separation process in which compressed gas is chilled in an ammonia separator so that it is partially condensed. The partially condensed stream is fed to a separator. The vapor component of the stream separated within the separator is introduced into a rectification column that utilizes a reflux condenser that again employs external ammonia refrigeration. U.S. Pat. No. 4,595,404 is another externally refrigerated process in which methane is separated from carbon dioxide by using a distillation column followed by a stripping column. The disadvantage of utilizing external refrigeration is that additional energy is consumed by such refrigeration and additional capital investment for the refrigeration system can represent an unacceptable economic penalty.
As will be discussed, the present invention provides an apparatus for separating carbon dioxide from a carbon dioxide feed stream that is inherently more thermally efficient than the prior art techniques and thus, consumes less compression energy, achieves high carbon dioxide recovery and further allows the carbon dioxide to be recovered at a high purity. Other advantages will become apparent from the following description of the present invention.